The present invention relates to a flash memory.
The flash memory is employed for a memory device such as a USB memory and a memory card. In addition to this, the flash memory is also mounted in a micro controller to store data used by the micro controller.
The flash memory is a nonvolatile memory and the rewritten data is stored in a memory cell. However, depending on the conditions after the data is rewritten, a threshold voltage in the memory cell may be varied significantly so that the data may not be properly read any longer.
For example, Patent Literature 1 discloses a nonvolatile memory comprised of a temperature sensor and a timer, in which the timer measures a period when the temperature measured with the temperature sensor exceeds a threshold temperature, and when the measured period reaches a threshold, refresh is performed.
As another example, Patent Literature 2 discloses equipment comprised of a nonvolatile memory. The nonvolatile memory is comprised of a timer to measure an elapsed time after performing a write operation to the nonvolatile memory, a temperature sensor to measure temperature near the nonvolatile memory, and a counter to count the number of times of the write operation to the nonvolatile memory. After weighting the period measured with the timer, based on the temperature measured with the temperature sensor and on the number of times of the write operation counted with the counter, a rewrite operation is performed to the nonvolatile memory when the weighted elapsed time measured with the timer exceeds a prescribed period.